estuarine residence times

Question 1

estuarine residence times

Answer 1

a.k.a Estuarine Flushing Times

• estuaries are not closed systems with rivers & tides flowing in & out of them
• flushing time = time to replace the freshwater volume of the estuary by the total freshwater input flux (river, discharges, rainfall etc.)
• info on estuarine functions -> nutrients, sediment, organic matter as well as waste & pollutants
• transport of organisms / larvae / spores

Question 2

Drogue buoy tracking

Answer 2

• sub-surface parachute allows buoy to drift with current
• tracked using GPS to determine pathway of a water parcel

Question 3

Rhodamine dye tracing

Answer 3

• used in conjunction with drogue buoys
• gives indication of water velocity but also mixing potential -> how dilute does dye patch become over time?
• dye goes opaque under UV light, allowing fluorimeter to measure concentrations as low as 1ppb
• dye turns skin pink for up to 2 weeks so wear
  PPE

Question 4

Acoustic Doppler Current Profiling

(ADCP)

Answer 4

• can be boat-mounted / placed on seabed frame
• using shift in reflected sound freq (aka Doppler shift) to determine water velocity at different depths
• transects across an estuary throughout a tidal cycle to calculate accurate flushing time

Question 5

Case Study - Tilbury Power Station, UK

Answer 5

• power stations are typically coastal to draw coolant water in from sea / discharge
  warmer water back into the sea / estuary
• flushing times needed so power stations know when to discharge coolant water or
  draw in new water. Avoid drawing warm waters back into coolant system / reactor
• warmer waters can help invasive sp take hold in an area

Question 6

Morecambe Bay, UK

Answer 6

• UK’s largest intertidal mudflats
• 5 rivers -> Leven, Kent, Keen, Wyre & Lune
• tidal speeds of up to 9 knots (“faster than a galloping horse”)
• low tide to high tide in 10 mins
• complex flushing time calculations due to extreme tidal dynamics
• flushing times needed to know optimum window for sewage discharge

Question 7

Venice Lagoon, Italy

Answer 7

• built in 6th C by Romans as a fortification
• barriers important to reduce storm surges
• pollution & introduction of invasive sp by visiting cruise ships
• residence time of up to 60 days (Cucco & Umgeisser, 2006)

Question 8

The Baltic Sea

Answer 8

• > 250 rivers feed into Baltic
• residence time of 30-40 years due to ‘pinch point’ at Denmark Straits
• long residence time & high concentration of urban environments / industry allows pollution & nutrients to accumulate => eutrification
• high dioxin conc from incineration & paper industry -> restrictions on fish for human
  consumption
• low O₂ (hypoxic) zones as O₂ not replenished

Question 9

assumptions of tidal prism method

Answer 9

• system is well-mixed
• tidal flow is dominent flushing mechanism
• system is at steady rate with sinusoidal tidal signal
• system is fully flushed in single tidal cycle